air (relative humidity = 25 to 30%) retained
90% of its initial PCE after 250 hours of con-
tinuous operation at MPP (Fig. 4H). The thermal
stability of the triple-halide material was also
tested by keeping opaque devices with gold
contacts at 85°C for 500 hours in nitrogen; the
unencapsulated triple-halide devices and double-
halide devices both retained >97% of their initial
efficiency (fig. S19). Semitransparent top cells
tested in N 2 exhibited negligible degradation
(<4%) after 1000 hours of continuous operation
at MPP near 60°C (Fig. 4H). This result con-
firmed the importance of encapsulation (N 2
in this case) and the role of barrier layers [an
ALD buffer layer and a transparent conduc-
tive oxide (TCO) layer in this case] to suppress
the reactions between metal contacts and
halides, as revealed in previous reports ( 73 – 75 ).
We integrated our triple-halide perovskite
in 1-cm^2 two-terminal monolithic tandems on
silicon cells (see photos in fig. S20). Here, the
ITO/glass substrate of the semitransparent
device (Fig. 4D) was replaced by a front-side
polished silicon heterojunction bottom cell
capped with a 20-nm-thick ITO recombination
layer, which allowed for solution processing of
the perovskite layer rather than vapor deposi-
tion that would be required to deposit the
perovskite on a pyramidally textured silicon
surface ( 65 , 76 ). With the aid of proper light
management, tandems on planar front-side
wafers have been shown to deliverJscvalues
competitive with those on fully textured wafers
( 77 ). Additionally, front-side planar substrates
are industrially relevant, because PERC (passi-
vated emitter and rear cell) silicon cells, which
constitute ~70% of the silicon PV market, have
a planar“shiny-etched”rear surface that was
recently shown to allow for solution process-
ing of the perovskite solar cell ( 78 ).
The full tandem structure is illustrated in
Fig. 5A. Figure 5B shows a cross-sectional SEM
image of the perovskite top cell on Si. Here,
we developed a NiOx/ poly-TPD bilayer hole
transport layer (HTL) for use in the tandems.
Relative to HTLs that used only spin-cast poly-
TPD, introducing a 20-nm-thick sputtered NiOx
layer between the ITO-HTL interface reducedshunting and increased the yield of 1-cm^2 tan-
dems. In aJ-Vsweep, the best-performing tan-
dem reached a PCE of 27.13% (Voc= 1.886 V,
Jsc= 19.12 mA cm–^2 , and FF = 75.3%). The
corresponding stabilized PCE at MPP is 27.04%
(Fig. 5C and Table 1), exceeding the current
record efficiency (26.7%) for Si single cells and
approaching the world record value of 28%
achieved by Oxford PV ( 15 , 16 ). TheVocof >1.88 V
is one of the highest reported in perovskite/Si
tandems with PCE of >25% ( 59 , 61 , 65 , 76 , 77 )
(see summary of≥1cm^2 – area tandems in table
S3), providing further evidence of the lowVoc
deficit achieved in the triple-halide semitrans-
parent top cells (table S2). The EQE spectrum
of the perovskite top cell matches well with
thosemeasuredintop-illuminationsemitrans-
parent devices (Fig. 5D). The active-areaJsc
values calculated from the EQE spectra were
19.3 mA cm–^2 and 19.9 mA cm–^2 for the perov-
skite top cell and Si bottom cell, respectively
(Fig. 5D). These calculated values indicate that
theJscof our two-terminal tandem was slightly
limited by the perovskite top cell.Xuet al.,Science 367 , 1097–1104 (2020) 6 March 2020 5of8
Fig. 3. Suppression of photoinduced phase segregation in triple-halide perovskites.(AandB) PL spectra of 1.67-eV control perovskite films (Cs25Br20) under
10-sun and 100-sun illumination for 20 min, respectively.Arrows indicate the direction of the PL shift over time. (C) The shift of the spectral centroids of control films over time.
The red shift becomes more obvious under higher injection. (DandE) PL spectra of 1.67-eV triple-halide perovskites (Cs22Br15+Cl3) under 10-sun and 100-sun illumination
for 20 min, respectively. (F) The shift of the spectral centroids of triple-halide perovskites over time. The blue shift becomes more obvious under higher injection level.
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